How to convert a jpg tp yuv in C?

Even though a question of this nature sounds very similar, I am having problems in converting a jpg image to yuv in C (without using opencv).

This is what I have understood as of now, how to solve this problem :

1. Identify the structure of file formats for jpg and yuv. i.e what each byte in the file actually contains. This is what I think jpg format looks like.
2. With the above structure I tried to read a jpg file and tried to decipher its 18th and 19th bytes. I did type cast them to both char and int but I don`t get any meaningful values for width and height of the image.

3. Once I have read these values, I should be able to convert them from jpg to yuv. I was looking at this resource.

4. Appropriately, construct yuv image and write it to a (.yuv) file.

Kindly help me by pointing me to appropriate resources. I will keep updating my progress on this post. Thanks in advance.

via Chebli Mohamed